As a crankshaft support structure, there has been known a structure in which a steel-made bearing cap is fixed to an aluminum alloy-made crankcase by bolts, and a crankshaft is rotatably supported between the crankcase and the bearing cap.
In the case where the crankcase and the bearing cap are fixed to each other by only bolts, the difference in material may lead to stress concentration being generated upon vibrations arising from the rotation of the crankshaft.
In relation to automobile engines, there has been disclosed a structure in which a bearing cap is press fitted in order to restrain the above-mentioned vibrations (see, for example, Patent Document 1). In addition, there has also been disclosed a structure in which a crankcase is formed with an arcuate recess in order to reduce stress concentration of loads arising from the rotation of the crankshaft (see, for example, Patent Documents 2 and 3).